Singlespeed & Fixed Gear"I still feel that variable gears are only for people over forty-five.
Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

so im planning on converting my 70s motobecane nomade into a single speed. it has rigida rims. how would i go about getting the cassette off since i can not find a freewheel puller that will work on it?

what kind of cassette is it? Finding the appropriate freewheel tool really helps. Did you try taking the wheel to your LBS? They tend to have lots of different freewheel tools. Since you just need it removed once you can have them bust it off and that will be the end of it. If that fails to produce results, go to park tool.com and open the repair help section. There are instructions for destructive free wheel removal(your freewheel is toast but it is off the wheel)

your wheel either has a cassette or a freewheel, they are not the same. since it's old it's almost certainly a freewheel. that means you can't take it apart and remove cogs like you describe, that only works with a cassette.

you can
a)take it off with the appropriate tool,
b)try to pry it off with something (probably wont work)
c)go to the park tool site and read about destructive freewheel removal,
d)just wrap the chain around whichever cog yields a straight chainline or
e)replace the wheel.
those are your options.